Mark it unlikely for Unref() to return true.

In the hot path, specially when the dtor of a class is inlined,
we will have to skip quite a few instructions (for Delete and dtor)
before returning.

Mark Unref() as unlikely so that compiler moves the
decrement-refcnt-and-return code instructions to the front.

src/core/ext/transport/chttp2/transport/internal.h

@@ -238,7 +238,7 @@ class Chttp2IncomingByteStream : public ByteStream {
   // switch to std::shared_ptr<>.
   void Ref() { refs_.Ref(); }
   void Unref() {
-    if (refs_.Unref()) {
+    if (GPR_UNLIKELY(refs_.Unref())) {
       grpc_core::Delete(this);
     }
   }

src/core/lib/gprpp/orphanable.h

@@ -110,12 +110,12 @@ class InternallyRefCounted : public Orphanable {
   }
 
   void Unref() {
-    if (refs_.Unref()) {
+    if (GPR_UNLIKELY(refs_.Unref())) {
       Delete(static_cast<Child*>(this));
     }
   }
   void Unref(const DebugLocation& location, const char* reason) {
-    if (refs_.Unref(location, reason)) {
+    if (GPR_UNLIKELY(refs_.Unref(location, reason))) {
       Delete(static_cast<Child*>(this));
     }
   }

src/core/lib/gprpp/ref_counted.h

@@ -211,12 +211,12 @@ class RefCounted : public Impl {
   // private, since it will only be used by RefCountedPtr<>, which is a
   // friend of this class.
   void Unref() {
-    if (refs_.Unref()) {
+    if (GPR_UNLIKELY(refs_.Unref())) {
       Delete(static_cast<Child*>(this));
     }
   }
   void Unref(const DebugLocation& location, const char* reason) {
-    if (refs_.Unref(location, reason)) {
+    if (GPR_UNLIKELY(refs_.Unref(location, reason))) {
       Delete(static_cast<Child*>(this));
     }
   }

src/core/lib/transport/metadata.cc

@@ -466,7 +466,7 @@ void grpc_mdelem_do_unref(grpc_mdelem gmd DEBUG_ARGS) {
     case GRPC_MDELEM_STORAGE_INTERNED: {
       auto* md = reinterpret_cast<InternedMetadata*> GRPC_MDELEM_DATA(gmd);
       uint32_t hash = md->hash();
-      if (md->Unref(FWD_DEBUG_ARGS)) {
+      if (GPR_UNLIKELY(md->Unref(FWD_DEBUG_ARGS))) {
         /* once the refcount hits zero, some other thread can come along and
            free md at any time: it's unsafe from this point on to access it */
         note_disposed_interned_metadata(hash);
@@ -475,7 +475,7 @@ void grpc_mdelem_do_unref(grpc_mdelem gmd DEBUG_ARGS) {
     }
     case GRPC_MDELEM_STORAGE_ALLOCATED: {
       auto* md = reinterpret_cast<AllocatedMetadata*> GRPC_MDELEM_DATA(gmd);
-      if (md->Unref(FWD_DEBUG_ARGS)) {
+      if (GPR_UNLIKELY(md->Unref(FWD_DEBUG_ARGS))) {
         grpc_core::Delete(md);
       }
       break;

src/core/lib/transport/transport.h

@@ -98,7 +98,7 @@ inline void grpc_stream_unref(grpc_stream_refcount* refcount,
 #else
 inline void grpc_stream_unref(grpc_stream_refcount* refcount) {
 #endif
-  if (refcount->refs.Unref()) {
+  if (GPR_UNLIKELY(refcount->refs.Unref())) {
     grpc_stream_destroy(refcount);
   }
 }